Nanometer fine particles (nano particles) have come to the front recently as a material for producing stable monotone fluorescent particles, a material of magnetic particles, or as a componential member of tunable light emitting diode, a single-particle transistor, a building block for superdense magnetic storage medium etc.
The demand of the nano particles is particularly increasing recently because of the enlargement of its usage along with the advance in technology. The typical materials of the nano particles are some metals such as gold, platinum, or nickel; some compounds, such as platinum iron, titanium oxide, zinc oxide, cadmium selenide, or zinc sulphide. Such nano particles of various materials are manufactured through various synthetic methods (manufacturing methods), such as a homogeneous precipitation method, hydrothermal crystallization method, organometallic route process, etc.
Meanwhile, the inventors of the present invention invented a method of accurately controlling the deposition condition (reaction condition), by which the fine particles of desired diameter are continuously manufactured (Kokai (Jpn. unexamined patent publication) No. 2003-225900). This method uses a reactor having a micro flow path, allowing control of reaction conditions, such as temperature, concentration, duration etc.
There also have been many developments of technology for producing composites fine particles by mixing some of the fine particles (M. Azad Malik et al., Chem. Mater. 2002, 14, 2004-2010). Combination of different kinds of particles allows not only improvement of the existing characteristics of the fine particles, but also provision of new functions.
For example, coating the fine particle of CdSe or InP with ZnS, which is larger in bandgap than the materials, creates ZnS coated particle. This ZnS coated fine particle has an effect of preventing the holes or electrons to be brought onto the defective surface of fine particle, thereby significantly improving fluorescence efficiency. Similarly, coating gold (Au) fine particle with silica creates silica coated gold particle. The composite particles are very easily dispersed in glass through sol-gel process or the like. With this advantage the particles are useful for preparation of nonlinear optical material.
In production of fine composite particles of nanometer order, particularly of the coated fine composite particles explained above, control of reaction condition is necessary to create a particle having a desired diameter. Typically, it is necessary to use appropriate kind of organic molecules of the material and/or the co-existing surfactant, or to control speed of decomposition of precursor of particle by changing the temperature/duration in the reaction process.
However, the method of controlling diameter of particle by changing the temperature/duration in the reaction process, the control is not easy, and also the quality of resulting particles may be not always good.
More specifically, the conventional particle-manufacturing apparatuses are not capable of accurate control of the temperature/duration in the reaction process in the manufacturing of coated composite particles. In other words, the accurate control in the reaction conditions is indispensable to accurate control of coating amount, but it cannot be achieved by the conventional manufacturing apparatuses.
Because of this, the reaction process in the manufacturing of fine composite particles is set longer than necessary on purpose in order to ensure more accurate control of reaction condition.
However, this sometimes requires a change in condition of the reaction system, and an excessively long reaction process causes an increase in particle size distribution due to Oswald maturation or the like. In the case of ZnS coated CdSe fine particles, the increase in particle size distribution means an increase in distribution of degree of quantum effect which depends on the particle diameter. This may therefore result in distribution in fluorescent spectrum (M. Azad Malik et al., Chem. Mater. 2002, 14, 2004-2010 etc.)
In view of this problem, a new technology has been demanded by which reaction condition, especially duration in reaction temperature, concentration of fine core particle and raw material for coating layer, and a time for keeping the fine core particles and the raw material for coating layer inside the micro flow path can be accurately controlled, allowing continuous production of fine composite particles with desired coating amount. Further, to produce a composite fluorescent nano particle having a semiconductor nano particle as its nucleus, there has been a demand of a technique by which coating time is reduced, and the coating amount becomes even. If this technique is complete, the increase in distribution in fluorescence spectrum will be easily avoided.
Further, the change in characteristic, such as a change in absorption band of the silica coated gold particles due to plasmon response in the surface of a cluster, depending on the thickness of coating, is often seen also in the composite fluorescent nano particle having a semiconductor nano particle as its nucleus. This necessity of control the characteristic of fine composite particles is another reason why the even coating is required.
The present invention is made in view of the foregoing problems, and an object is to provide a manufacturing method and manufacturing apparatus for continuous manufacturing of fine composite particles, by which reaction condition is accurately controlled and coating distribution becomes even. The present invention also provides one kind of fine composite particles obtained by the manufacturing method.